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Crystal structure of bovine chymotrypsin. The catalytic residues are shown as yellow sticks. Rendered from PDB 1CBW.
Serine proteases (or serine endopeptidases) are enzymes that cleave peptide bonds in proteins, in which serine serves as the nucleophilic amino acid at the (enzyme's) active site. They are found ubiquitously in both eukaryotes and prokaryotes. Serine proteases fall into two broad categories based on their structure: chymotrypsin-like (trypsin-like) or subtilisin-like. In humans, they are responsible for co-ordinating various physiological functions, including digestion, immune response, blood coagulation and reproduction.
There are at least 14 families of serine protease. The majority belong to the S01 family.
Chymotrypsin-like serine proteases are characterised by a distinctive structure, consisting of two beta-barrel domains that converge at the catalytic active site. These enzymes can be further categorised based on their substrate specificity as either trypsin-like, chymotrypsin-like or elastase-like.
Trypsin-like proteases cleave peptide bonds following a positively charged amino acid (lysine or arginine). This specificity is driven by the residue which lies at the base of the enzyme's S1 pocket (generally a negatively charged aspartic acid or glutamic acid).
The S1 pocket of chymotrypsin-like enzymes is more hydrophobic than in trypsin-like proteases. This results in a specificity for medium to large sized hydrophobic residues, such as tyrosine, phenylalanine and tryptophan.
Subtilisin is a serine protease in prokaryotes. Subtilisin is evolutionarily unrelated to the chymotrypsin-clan, but shares the same catalytic mechanism utilising a catalytic triad, to create a nucleophilic serine. This is the classic example used to illustrate convergent evolution, since the same mechanism evolved twice independently during evolution.
Catalytic mechanism 
The main player in the catalytic mechanism in the chymotrypsin and subtillisin clan enzymes mentioned above is the catalytic triad. The triad is located in the active site of the enzyme, where catalysis occurs, and is preserved in all serine protease enzymes. The triad is a coordinated structure consisting of three essential amino acids: histidine (His 57), serine (Ser 195) (hence the name "serine protease") and aspartic acid (Asp 102). Located very near one another near the heart of the enzyme, these three key amino acids each play an essential role in the cleaving ability of the proteases.
In the event of catalysis, an ordered mechanism occurs in which several intermediates are generated. The catalysis of the peptide cleavage can be seen as a ping-pong catalysis, in which a substrate binds (in this case, the polypeptide being cleaved), a product is released (the N-terminus "half" of the peptide), another substrate binds (in this case, water), and another product is released (the C-terminus "half" of the peptide).
Each amino acid in the triad performs a specific task in this process:
- The serine has an -OH group that is able to act as a nucleophile, attacking the carbonyl carbon of the scissile peptide bond of the substrate.
- A pair of electrons on the histidine nitrogen has the ability to accept the hydrogen from the serine -OH group, thus coordinating the attack of the peptide bond.
- The carboxyl group on the aspartic acid in turn hydrogen bonds with the histidine, making the nitrogen atom mentioned above much more electronegative.
The whole reaction can be summarized as follows:
- The polypeptide substrate binds to the surface of the serine protease enzyme such that the scissile bond is inserted into the active site of the enzyme, with the carbonyl carbon of this bond positioned near the nucleophilic serine.
- The serine -OH attacks the carbonyl carbon, and the nitrogen of the histidine accepts the hydrogen from the -OH of the [serine] and a pair of electrons from the double bond of the carbonyl oxygen moves to the oxygen. As a result, a tetrahedral intermediate is generated.
- The bond joining the nitrogen and the carbon in the peptide bond is now broken. The covalent electrons creating this bond move to attack the hydrogen of the histidine, breaking the connection. The electrons that previously moved from the carbonyl oxygen double bond move back from the negative oxygen to recreate the bond, generating an acyl-enzyme intermediate.
- Now, water comes in to the reaction. Water replaces the N-terminus of the cleaved peptide, and attacks the carbonyl carbon. Once again, the electrons from the double bond move to the oxygen making it negative, as the bond between the oxygen of the water and the carbon is formed. This is coordinated by the nitrogen of the histidine, which accepts a proton from the water. Overall, this generates another tetrahedral intermediate.
- In a final reaction, the bond formed in the first step between the serine and the carbonyl carbon moves to attack the hydrogen that the histidine just acquired. The now electron-deficient carbonyl carbon re-forms the double bond with the oxygen. As a result, the C-terminus of the peptide is now ejected.
Additional stabilizing effects 
It was discovered that additional amino acids of the protease, Gly 193 and Ser 195, are involved in creating what is called an oxyanion hole. Both Gly 193 and Ser 195 can donate backbone hydrogens for hydrogen bonding. When the tetrahedral intermediate of step 1 and step 3 are generated, the negative oxygen ion, having accepted the electrons from the carbonyl double bond fits perfectly into the oxyanion hole. In effect, serine proteases preferentially bind the transition state and the overall structure is favored, lowering the activation energy of the reaction. This "preferential binding" is responsible for much of the catalytic efficiency of the enzyme.
Regulation of Serine Protease activity 
Host organisms must ensure that the activity of serine proteases is adequately regulated. This is achieved by a requirement for initial protease activation, and the secretion of inhibitors.
Zymogen activation 
Zymogens are the usually inactive precursors of an enzyme. If the digestive enzymes were active when synthesized, they would immediately start chewing up the synthesizing organs and tissues. Acute pancreatitis is such a condition, in which there is premature activation of the digestive enzymes in the pancreas, resulting in self-digestion (autolysis). It also complicates postmortem investigations, as the pancreas often digests itself before it can be assessed visually.
Zymogens are large, inactive structures, which have the ability to break apart or change into the smaller activated enzymes. The difference between zymogens and the activated enzymes lies in the fact that the active site for catalysis of the zymogens is distorted. As a result, the substrate polypeptide cannot bind effectively, and proteolysis does not occur. Only after activation, during which the conformation and structure of the zymogen change and the active site is opened, can proteolysis occur.
|Trypsinogen||trypsin||When trypsinogen enters the small intestine from the pancreas, enteropeptidase secretions from the duodenal mucosa cleaves the lysine 15 - isoleucine 16 peptide bond of the zymogen. As a result, the zymogen trypsinogen breaks down into trypsin. Recall that trypsin is also responsible for cleaving lysine peptide bonds, and thus, once a small amount of trypsin is generated, it participates in cleavage of its own zymogen, generating even more trypsin. The process of trypsin activation can thus be called autocatalytic.|
|Chymotrypsinogen||chymotrypsin||After the Arg 15 - Ile 16 bond in the chymotrypsinogen zymogen is cleaved by trypsin, the newly generated structure called a pi-chymotrypsin undergoes autolysis (self digestion), yielding active chymotrypsin.|
|Proelastase||elastase||It is activated by cleavage through trypsin.|
As can be seen, trypsinogen activation to trypsin is essential, because it activates its own reaction, as well as the reaction of both chymotrypsin and elastase. Therefore, it is essential that this activation does not occur prematurely. There are several protective measures taken by the organism to prevent self-digestion:
- The activation of trypsinogen by trypsin is relatively slow
- The zymogens are stored in zymogen granules, capsules that have walls that are thought to be resistant to proteolysis.
There are certain inhibitors that resemble the tetrahedral intermediate, and thus fill up the active site, preventing the enzyme from working properly. Trypsin, a powerful digestive enzyme, is generated in the pancreas. Inhibitors prevent self-digestion of the pancreas itself.
Serine proteases are inhibited by a diverse group of inhibitors, including synthetic chemical inhibitors for research or therapeutic purposes, and also natural proteinaceous inhibitors. One family of natural inhibitors called "serpins" (abbreviated from serine protease inhibitors) can form a covalent bond with the serine protease, inhibiting its function. The best-studied serpins are antithrombin and alpha 1-antitrypsin, studied for their role in coagulation/thrombosis and emphysema/A1AT, respectively. Artificial irreversible small molecule inhibitors include AEBSF and PMSF.
Role in disease 
Mutations may lead to decreased or increased activity of enzymes. This may have different consequences, depending on the normal function of the serine protease. For example, mutations in protein C, when leading to insufficient protein levels or activity, predispose to thrombosis.
Diagnostic use 
Determination of serine protease levels may be useful in the context of particular diseases.
- Coagulation factor levels may be required in the diagnosis of hemorrhagic or thrombotic conditions.
- Fecal elastase is employed to determine the exocrine activity of the pancreas, e.g., in cystic fibrosis or chronic pancreatitis.
- Serum prostate-specific antigen is used in prostate cancer screening, risk stratification, and post-treatment monitoring.
- Serine protease, as released by mast cells, is an important diagnostic marker for type 1 hypersensitivity reactions (e.g. anaphylaxis). More useful than e.g. histamine due to the longer t1/2, meaning it remains in the system for a clinically useful length of time.
- Hedstrom, L. (Dec 2002). "Serine protease mechanism and specificity.". Chem Rev 102 (12): 4501–24. doi:10.1021/cr000033x. PMID 12475199.
- Madala PK, Tyndall JD, Nall T, Fairlie DP. (Jun 2010). "Update 1 of: Proteases universally recognize beta strands in their active sites". Chem Rev 110 (6): PR1–31. doi:10.1021/cr900368a. PMID 20377171.
- Ovaere P, Lippens S, Vandenabeele P, Declercq W. (Aug 2009). "The emerging roles of serine protease cascades in the epidermis". Trends Biochem Sci 34 (9): 453–63. doi:10.1016/j.tibs.2009.08.001. PMID 19726197.
- Evnin, Luke B.; Vásquez, John R.; Craik, Charles S. (1990). "Substrate specificity of trypsin investigated by using a genetic selection". Proceedings of the National Academy of Sciences of the United States of America 87 (17): 6659–63. doi:10.1073/pnas.87.17.6659. JSTOR 2355359. PMC 54596. PMID 2204062.
- Kimball's Biology Pages, Serine Proteases[self-published source?]
See also 
- The MEROPS online database for peptidases and their inhibitors: Serine Peptidase
- Serine Proteases site at Saint Louis University (SLU)
- Serine proteases at the US National Library of Medicine Medical Subject Headings (MeSH)